This invention relates to a torque detector with which the torque that is applied as an external force to a driven shaft such as a rotating shaft can be detected in a non-contact fashion.
In automotive areas such as the power steering mechanism, anti-skid braking mechanism and automatic transmission control, a need often occurs to detect the torque as it is applied to driven shafts such as the shaft of a steering wheel. An example of the torque detectors that are to be used in those applications is a magnetostrictive torque detector that has been disclosed in Unexamined Published Japanese Patent Application (kokai) Hei-1-94230. The construction of this detector is described below with reference to FIG. 10.
In the drawing, numeral 1 denotes a driven shaft which is a rotating shaft; 7a and 7b are bearings that support the driven shaft 1 rotatably; and 3 is a bobbin that is also supported by the bearings. A first magnetic member 2a and a second magnetic member 2b, each being made of a layer of a magnetostrictive material, are secured to the circumferential surface of the driven shaft 1 as they are axially spaced from each other. The first magnetic member 2a is formed as a plurality of thin stripes that are arranged at an angle (.theta.) of 45.degree. with respect to the center axis whereas the second magnetic member 2b is formed as a plurality of thin strips that are arranged at an angle (.theta.) of -45.degree.. The bobbin 3 is fitted with a first coil 5a and a first yoke 4a in association with the first magnetic member 2a, as well as a second coil 5b and a second yoke 4b in association with the second magnetic member 2b. The yokes 4a and 4b are members that prevent magnetic fluxes from spreading towards the outside. Shown by 100 is a stress detecting circuit that is connected to the first and second coils 5a and 5b.
The torque detector shown in FIG. 10 operates as follows. When an external torque is applied to the driven shaft 1, a main stress develops on the surface of the driven shaft in the directions defined by .theta.=.+-.45.degree., whereupon a tensile component of the stress works on one of the magnetic members 2a and 2b and a compressive component of the stress works on the other. If this stress is created, the permeability of the magnetic members 2a and 2b will change and the direction of the change that occurs upon development of the tensile stress is opposite to the direction for the case where the compressive stress develops. The stress detecting circuit 100 detects the inductances of the coils 5a and 5b that develop in response to the changes in the permeability of the magnetic members, calculates the torque that has been applied to the driven shaft 1, and outputs a voltage proportional to the torque.
The thus constructed magnetostrictive torque detector has had the following problems. First, the provision of magnetic members 2a and 2b as a plurality of strips on the surface of the driven shaft 1 requires a complex procedure; furthermore, the magnetic members in the form of strips will come off easily and corrosion has occasionally developed at the boundary between the shaft 1 and each magnetic member.
In the magnetostrictive torque detector under consideration, the changes in permeability that occur in opposite directions in the magnetic member 2a disposed in the direction of +45.degree. and the magnetic member 2b disposed in the direction of -45.degree. are detected via different coils 5a and 5b; although this is effective in compensating for the temperature characteristics of the magnetic members, external magnetic field and residual magnetic flux; various disturbances such as an axial temperature gradient, unbalanced external magnetic field and residual magnetic flux cannot be fully compensated. For example, the magnetic members 2a and 2b are axially spaced from each other, so if the driven shaft 1 has an axial temperature gradient, the magnetic members can have different temperatures that will eventually cause an error in the result of inductance detection.